Allogeneic hematopoietic cell transplantation following non- myeloablative conditioning is a procedure with extraordinary potential. At the same a time better understanding of the composition of the hematopoietic graft itself, particularly with respect to facilitative cell populations; continues to make prospects for transplantation of highly purified hematopoietic stem cells (HSCs) increasingly enticing. This proposal seeks to combine these two areas of rapid development into a significant research proposal. Using the murine model, this proposal entails efforts to: (1) develop and mechanistically study a clinically relevant model of non-myeloablative allogeneic hematopoietic cell transplantation based on a conditioning regimen of sublethal irradiation and an anti-CD3 monoclonal antibody followed by transplantation with rigorously purified HSCs, in experiments that will model minor histocompatibility, haplo-identical, and fully allogeneic barriers, (2) further current understanding of a facilitative cell population that enhances engraftment of purified HSCs with careful studies into this phenomenon in the setting of minor histocompatibility antigen disparities, and (3) combine lessons learned from both efforts to fully explore the role of engraftment facilitation as another modality for reducing conditioning requirements for donor hematopoietic engraftment and/or enhancing donor chimerism particularly within the T cell compartment. An important component of allogeneic hematopoietic cell transplantation is the conferral of a graft-versus-- malignancy effect, and the facilitator-optimized non-myeloablative transplantation approach developed with this proposal will be rigorously evaluated within the context of a tumor model. Lastly, an additional goal of this proposal is to better understand the barrier to engraftment in all of these settings, including facilitated versus non- facilitated engraftment of purified HSCs, following myeloablative as well as reduced intensity conditioning, and across a range of genetic disparities and engineered attenuation of the engraftment barrier.